3min 15 sec

[rcflyertim]

Ok...I don't understand what is significant about 3 min and 15 sec. for marking time. Can someone explain.

With the 3:15 method you can count out very accurately the speed and heading of the target. It's quite simple.
The speed of a ship in knots equals with the distance she travels in km multiplied by 10. E.g. When you sight the target, mark her position on the map and start your chronometer. At 3 min 15 sec mark the target's position again and measure the distance between the first and the second marking. If it reads 0.9 km, multiply it by 10 and you'll get 9 knots, i.e. the speed of the target. If you connect the markings with a line, you'll also get the direction of the target.

[Pisces]

3m 15 seconds is close enough to 194.4 seconds. That is the time a ship moving at 1 knots takes to move 100 meters. 100m being the smallest unit of measure that the ruler tool can show on the map. (as a number)

The idea is to plot positions with this interval, and then the number of 100 meter units is the speed of the target. 600 meters between points means 6 knots, 1300 meters means 13 knots. It was derived from a similar rule where 1 knots takes roughly 3 minutes to move 100 yards.

But personally I would advise to take the average of 13 minutes (4x 3m15s) as then the speed would be a bit more accurate. (Divide by 4 ofcourse) As there will allways be some inaccuracy in where you place the plotmark, and the source that provides the location info. 6 knots, give or take 1, could result in a miss if the target isn't at point blank range. 6 knots, give or take 0.25 knots is much more reliable.

[palatum]

Wouldnt the most accurate be (194.4 seconds x 4) = 12 mins 57 seconds?
I dont know how much 3 seconds means, but it's a difference never the less.

[HundertzehnGustav]

depends...
of course, the longer one observes its prey, the better the calcs are.
Now at math i suxxorx, but how much difference does a 3 second difference make... how far off will your shot be, if a ship is 50 to 150 metres long, makes 6 kts, and moves for 3 seconds?

guess its not worth the hassle, considering that you have to keep your eye on your prey, gauge the distance accurately too, and do it in not-so-nice weather as well?

if such a long observation period is needed, i suppose the ship is either making high speed, (difficult shot) or far away (difficult shot)
... in both cases, or maybe if both factors are combined, a torp shot is not likely to hit, right? and you do not want to send two or three eels, hoping one hits...

Then you better surface and run ahead of the bugger, at the presumed course.


(...??)

[rcflyertim]

Ahhh so it is a knots thing. OK so 3 min 15 sec x 4 = 13 min. If I wait 13 minutes I will miss my target! So I take it that these times are for long distance spottings?

[Bubblehead729]

You don't necessarily have to wait the whole 13 minutes; its just that the longer time tends to reduce the amount of error in the speed calculation. Just remember the 3:15 time interval (for metric units-3:00 for Imperial units).
So if you have a limited amount of time, just time for 3:15; if you have longer, then take multiple measurements every 3:15.

[Pisces]

The 3 seconds difference from 13 minutes can be neglected for simplicity. It's only 0.4% off from exact (777.5 seconds, or 12m57s). Even 195 seconds (3m15s) is 0.3% from exact (194.4s) So, you give up the same amount of inaccuracy on your speed for either time-interval. This is not the reason why it is more accurate.

The idea behind taking the target motion over 4x 3m15s, instead of just 1x3m15s, is spreading the position error of the begin and endpoint over a longer time period.

Let's say your watchofficer on the bridge gives a target position report. He gives distances in steps of 100 meters, and a bearing in steps of degrees. A degree at maximum visual distance of 16km is 280 meters wide. So the beginpoint of the target motion is somewhere in such a 100m by 280m box on the map. If you are lucky it is smaller when the target is closer. But doing periscope observations is much more inaccurate anyway.

The target speed is a line connecting some point in the begin-box with the a point in the end-box, taken over the time period. There is a smallest distance connecting the inside borders of the boxes. But the speed can also be the largest distance, connecting the far edges of the boxes. The actual motion of the target is somewhere in between those lengths.

If the interval is really short (theoretical 1 second) then the position uncertainty totally envellops the target motion track. If the interval is much longer, then the position uncertainty is only a small fraction of the target motion. A 6 knot target moves 600 meters in 3m15s. Two 280m position uncertainty boxes spaced appart by 600m would suggest a speed of something between 3.2 and 8.8 knots. That's quite a range. Conclusion: 3m15s is useless considering position inaccuracy in the plots.

However, it moves 2.4 kilometer in 13 minutes. The uncertainty boxes are still of the same size, but are much further appart. Based on the uncertainty boxes, it could have a speed between 5,3 and 6.7 knots. Now you can be much more confident that it is actually moving with 6 knots! It could still be 5.5 knots, or 6.5 knots. But you certainly narrowed it down to something acceptable.